Retroreflective materials are used in everyday life and may be seen in road surfaces, road signs, vehicles, and clothing. Retroreflective materials are configured to reflect light, or other forms of radiation, back to an originating source regardless of an angle of incidence.
One common type of retroreflector is provided by a surface of cube corners, or microprisms. Two types or cube corners are typically utilized, a full cube corner and a triangular cube corner. A typical full cube corner has three square facets and a hexagonal aperture.
FIG. 1A illustrates the reflective properties of a full cube corner 101. A typical full cube corner has three square facets 103, 105, and 107. The full cube corner 101 has a hexagonal aperture 109 that is also the effective area of the full cube corner when incident light is in a 0° entrance angle. The incident light within the effective area is constructively reflected by three facets 103, 105, and 107 and may pass through the effective area again so as to goes back to the incident direction.
A full cube corner 101 may be best suited for applications in which an angle of entrance, or an angle of incident light, is between 0°-30°. In the 0°-30° angle of incident light range, the entire inner cube surface may behave as a retroreflector, where the entire hexagonal aperture may be regarded as a retroreflective area and substantially all of the incident light will be retro-reflected by three internal reflections of three facets. Therefore, any incoming light 102 entering the full cube corner through the aperture within an angle of incident light range of 0°-30° may be retro-reflected regardless of where the incoming light 102 strikes the full cube corner 101. This enables the full cube corner to reflect light 104 in a substantially parallel path in comparison to the incident light 102, with approximately 0°-2° of deviation. The deviation of the retroreflected light may depend on any deviations associated with the 90° formed by the intersection of the three facets 103, 105, and 107 of the full cube corner. Retroreflective articles formed with molds comprising full cube corner 101 microstructures may be ideal for certain situations. For example, full cube corner retroreflectors may be ideal for retroreflective articles intended to be used on items such as highway signs, automobiles, or clothing where a person may likely be viewing reflected light from these items within the observation angle of 0°-2° at the entrance angle range of 0°-30°.
In contrast, FIG. 1B illustrates the reflective properties of a triangular cube corner 111. A typical triangular cube corner may include three right angle isosceles facets 113, 115, and 117 and may have an equilateral triangular aperture. Because the triangular cube corner is not symmetrical, the effective area of the triangular cube corner, through which the 0° incident light is retro-reflected by three facets 113, 115, and 117, may be defined by a hexagonal area 119. The hexagonal area 119 is approximately ⅔ of the triangular aperture of the triangular cube corner. The remaining ⅓ of the triangular aperture is known as a redundant area.
The triangular cube corner 111 may be better suited for other applications in which an angle of incident light is in the range of 30°-60°, where the redundant area may be added to be active in the retro-reflection of light. The effective area of the triangular cube corner may be larger than that of the full cube corner at entrance angle range of 30°-60°. The triangular cube corners 101 may be ideal for situations where the retro-reflected light is desired to be observed at entrance angle 30°-60° range. An example of such a situation may include a motorist viewing the light reflected from a large overhead sign, highway signs, and personal safety.
The cube corners of either type may be formed, in one method, by a mold having a surface with the microstructure of the desired shape. U.S. Pat. No. 6,015,214 issued to Heenan et. al., on Jan. 18, 2000, and assigned to Stimsonite Corporation (herein referred to as Heenan), describes a first method for forming microcube molds used in providing retroreflective articles. This method utilizes a number of plates, or shims, stacked together. A diamond cutting tool can than be used to form a set of 90° v-shaped grooves on the top surface of the plate stack. Alternating individual plates are then shifted to provide a full cube corner configuration. While the resulting shifted cube corner configuration contains three exposed facets, only two of the three facets are smooth enough to be reflective (see Heenan, FIGS. 3-5).
Heenan further describes a second method for forming the molds used in making retroreflective articles. In this second method, a retroreflective cube corner surface is formed by cutting v-shaped grooves in a stack of plates and adjoining alternating plates rotated 180°. The configuration of the microcubes in the second method results in the elimination of the exposed non-reflective facets (Heenan, FIGS. 18-21).
The methods of diamond turning are also illustrated in FIGS. 1 and 2 of U.S. Pat. No. 6,206,525 issued to Rowland et. al., on Mar. 27, 2001, and assigned to Reflexite Corporation. Additional illustrations of non-pin based methods of diamond turning may also be found in FIG. 3 of U.S. Pat. No. 6,626,544 issued to Lu et. al., on Sep. 30, 2003, and also assigned to Reflexite Corporation.
A third known method of forming molds used in the production of retroreflective articles involves the use of bundles of pins. First, each individual pin of a “pin bundle” are separately machined in the shape of a desired cube corner. The separately machined pins are then bundled together to form a microcube surface configuration featuring a collection of cube corners. The microcube surface configuration may thereafter be used to form a mold.